The invention relates to an apparatus and method for producing filaments from a thermoplastic polymer, and in particular relates to a self-texturing filament formed from polyester that exhibits a desirable tendency to form helical coils when properly drawn and finished rather than remain straight and without texture. The apparatus and method are particularly well suited for forming spiraled high denier hollow filaments from recycled materials.
Synthetic polymers are used in many textile applications to replace natural textile materials such as wool and cotton. Synthetic polymers are also used for other textile-related applications such as insulation layers in clothing, particularly clothing for outdoor use in colder weather, and for bulking properties in pillows and other such products in which these properties are alternatively provided by natural materials such as feathers or by synthetic foam materials.
The starting product for almost all synthetic textile material is a liquid polymer. The liquid polymer is extruded through a device called a xe2x80x9cspinneretxe2x80x9d containing at least one and typically many small orifices. Extruding the liquid polymer in this fashion creates extended solid cylindrical filaments. Such filaments have some immediate uses such as fishing line. In textile applications, however, synthetic filaments and the fibers and yarns made from them should desirably provide properties similar to those of natural fibers such as wool or cotton. In order to provide such properties, synthetic filaments must be modified or textured before being formed into yarns and fabrics. As is well understood in the textile industry, texturing can comprise crimping, looping, or otherwise modifying continuous filaments to increase their cover, resilience, abrasion resistance, warmth, insulation properties, and moisture absorption, or to provide a somewhat different surface texture. Filaments are also structurally modified to impart desired physical properties. For example, hollow filaments of cylindrical and triangular cross-sections or possessing bulbous appendages are known in the art.
Typical texturing methods include false twist texturing, mechanical texturing such as edge crimping or gear crimping, air jet crimping, knit-de-knit crinkles, or core-bulked filaments. Each of these has its own particular properties, advantages, and disadvantages.
Among these various types of textured filaments, coils are preferred for certain applications such as cushions and insulation. Coiled filaments tend to give more volume and fewer sharp bends, xe2x80x9czing-zags,xe2x80x9d or xe2x80x9cknees.xe2x80x9d Generally speaking, coiled filaments take on a coil or spiral configuration that is somewhat more three dimensional than other textured filaments and thus are preferred for many bulking applications, including those mentioned above. Hollow coiled filaments are particularly useful in bulking applications.
Typical methods for coiling filaments include false twisting or edge crimping, both of which techniques are well-known to those of ordinary skill in the art, and will not be otherwise further described herein. Both of these methods have various advantages and disadvantages in producing coiled yarns. For example, false twist coiling requires a conventional false twist winding system, while an edge crimp method requires the mechanical devices necessary to physically produce the crimp.
Alternatively, coiled filaments can be formed from bilateral fibers that coil following further processing. Traditionally, bilateral fibers are formed from two different generic fibers or variants of the same generic fiber extruded in a side-by-side relationship. Although side-by-side or xe2x80x9cbicomponentxe2x80x9d spinning offers certain advantages, it also is a relatively demanding process that requires more complex spinning equipment and thus is advantageously avoided where unnecessary.
In the early and mid-1990""s, apparatus and methods for forming hollow self-texturizing filaments were developed that avoided the problems associated with the above-mentioned texturing methods. These apparatus and methods utilized, among other things, a unique quenching method that coiled the filament without mechanical manipulation of the filament. U.S. Pat. Nos. 5,407,625; 5,510,183; and 5,531,951 (commonly assigned to the present assignee) are representative of this step forward in textile technology.
In the past few years, a combination of the public""s increasing awareness of the finite nature of natural resources, the desire to reduce pollution and improved recycling technology has greatly expanded the market for recycled materials. Accordingly, manufacturers continue to search for new and better methods for incorporating recycled materials into their production processes. The polymer industry is particularly active in this area.
Many companies currently use recycled polymers (i.e., polyester) to manufacture various goods including filament. Nevertheless, the use of recycled polyesterxe2x80x94the majority of which is from post-consumer beverage bottlesxe2x80x94as feedstock for filament production poses numerous problems. One problem associated with recycled feedstock (as opposed to virgin polymer) is the variation in the viscosity (which directly reflects the molecular weight) of the feedstock. Another problem is the presence of unwanted contaminants. Both of these problems disturb the flow of the melt through the spinneret, which in turn leads to disruptions in the subsequent processing of the filament of which the most troublesome are line breakages.
The above problems may be reduced or eliminated by (1) improving the quality of recycled feedstock, or (2) adjusting the production process. By its very nature, the quality of feedstock is often governed by factors outside of the control of the recycler. Accordingly, adjustment of the production process for a wide variety of feedstocks often offers the most promising long term means to address these problems.
One typical adjustment is to use filtration to remove a portion of the contaminants from the recycle. Filter life depends upon the filter fineness (i.e., grade) and the percentage of contaminants removed. Long filter life is desired to avoid process interruptions therefore coarser filters are often used. Coarser filters allow more contaminants to pass through. Accordingly, additional contaminant related adjustments, such as the use of larger spinneret holes (which allow passage of larger contaminant particles) are often necessary. Another typical adjustment is to use a spinneret with fewer orifices at the same throughput thereby producing larger denier filament (i.e. 6-15 dpf). The larger denier can be achieved via slower take-up speeds and higher throughput per hole. The larger orifices also provide more area for passage of solid contaminants. Changing size, of course, can limit certain end uses for the filaments.
Processes upstream and downstream of the spinneret, however, are typically fixed for a specific volume of product. For example, the quantity of polymers fed to the spinneret and the take-up speed of the filament (i.e., the speed at which the filament is taken up on rollers) are usually fixed or are only minimally variable. Thus, a combination of fixed volume and larger orifice translates to slower velocities through the spinneret (i.e., slower extrusion speeds). For regular filament the ratio of take-up speed (or process speed) to extrusion speed is typically on the order of 120:1. This ratio is often called the stretch ratio because the change in velocity physically stretches the filament.
stretch ratio=process speed÷extrusion speed
For large denier hollow-fiber filament, which has a slower extrusion speed, the stretch ratio is typically around 300-400:1.
The high stretch ratio for production of large denier hollow-fiber filament causes problems in the production of spiral filaments. The spiral effect is achieved through use of a rapid one-sided quench as described in the previously listed and commonly assigned patents. The rapid quench combined with a slow extrusion speed and high stretch ratio increases the maximum value of the melt""s strain rate as the filament is formed in the quench zone. The maximum strain rate, also called the xe2x80x9cmaximum dv/dxxe2x80x9d, is the maximum value of the ratio
dv/dx=increase in fiber speed in the process direction incremental distance in the process direction
This ratio only applies where the fiber is still molten in the quenching zone and increasing in velocity from the extrusion velocity to the take up velocity. If the maximum strain rate is exceeded the filament breaks which interrupts production.
In other words, because the filament solidifies shortly after leaving the spinneret the filament must rapidly accelerate from the slow extrusion speed to the fast take-up speed. This rapid acceleration places a large amount of strain on the filament. If the strain rate is higher than the polymer relaxation time the filament will likely break. Slower cooling (i.e., slower quench) can moderate this problem but a faster differential cooling is needed to spiral the filament as noted in the commonly assigned patents.
In short, current production methods for large denier hollow fiber are hindered by opposing but unavoidable production components: rapid quenching and high stretch ratios. Therefore, a need exists for an apparatus and method for producing self-texturing hollow fiber filament from recycled material and particularly higher-denier filament from recycled material that avoids the difficulties associated with current production practices.
Accordingly, an object of the present invention is to develop an apparatus and method for forming hollow fiber that avoids the problems associated with current production techniques. In particular, an object of the present invention is to develop an apparatus and method for forming hollow fiber from recycled material that avoids the production problems associated with extruding feed material defined by varying viscosity and solid contaminants.
The invention meets this object with a spinneret for the production of hollow filament comprising first and second non-linear (e.g. curved slots). A first end having a first width and a second end having a second, different width define in part each slot. An intermediate portion defined in part by a graduating width along its length separates the first and second ends of each slot.
In another aspect, the invention is a method of producing hollow filaments comprising extruding a polymer melt through a spinneret where the spinneret comprises first and second non-linear (e.g., curved slots) where each slot has a first end having a first width and a second end having a second, different width. Each slot is further defined as having a graduating width along the continuum defined by the distance between the first and second ends of each slot.
Alternatively, the invention may be described as an extrusion characterized by a velocity gradient along the length of the openings through which polymer melt is extruded. For ease of discussion, the invention is discussed in the context of hollow cylindrical filaments which are a commonly produced hollow filament. Those skilled in the art, however, will readily recognize that the concepts of the invention, particularly extruding polymer melts having a graduated width, are equally applicable to non-cylindrical hollow filaments.